Castelia Sewers (Japanese: ヒウン下水道Hiun Sewers) is a location in south Unova. It is accessible via an entrance in Castelia City, on the pier near the Bridge Gate. When exploring for the first time, Hugh will join the player as a tag partner. Here the player meets Colress for the first time. Relic Passage is accessible from here. There is also an exit to Castelia City's park with wild Pokémon, and when the sewers are flooded, to Castelia City's back alley. During autumn and winter, the sewers are dry, blocking some areas and removing the requirement of Surf to access others.

Scientists

Once per day, a female Scientist in a room near the Doctor and a male Scientist near the Janitor with Marill and Cinccino are attempting to turn poison into medicine will give the player an item created in their experiment. They can give out any one of these items on a daily basis.

In the anime

Castelia Sewers in the anime.

The Castelia Sewers makes an appearance in A Venipede Stampede!. Burgh, Ash and his friends observe a Venipede stuck in one of the pipes and the Venipede gets freed with their help. Notably, this area appeared in the anime long before it was accessible in the games.